An internal combustion engine includes a valve drive mechanism for driving an intake valve and an exhaust valve. Abnormality detection devices disclosed, for instance, in Patent Documents 1 to 4, detect a malfunction of the intake valve or exhaust valve, that is, an abnormality of the valve drive mechanism.
The device disclosed in Patent Document 1 includes a lift sensor that detects the lift amount of the intake valve or exhaust valve, which may be hereinafter collectively referred to as an “intake/exhaust valve.” The device disclosed in Patent Document 1 directly detects a malfunction of an intake/exhaust valve in accordance with an output of the lift sensor. The device disclosed in Patent Document 2 detects a failure of a cylinder halt mechanism, which halts the drive of the intake/exhaust valve, in accordance with an intake air amount. The device disclosed in Patent Document 3 indirectly detects a malfunction of the intake/exhaust valve in accordance with an exhaust gas sensor output.    Patent Document 1: JP-A-2004-100487    Patent Document 2: JP-A-2005-139962    Patent Document 3: JP-A-2004-100486    Patent Document 4: JP-A-11-141364